For products that communicate audio signals from one location to another location, for example, a full-duplex speakerphone, some of the acoustic energy from the far-end voice signal will transfer into the microphone that picks up the near-end voice signal. If a fraction of the far-end voice energy transfers into the near-end speech path, a person on the far end of the call will hear their own voice returned as an annoying echo.
A speakerphone includes a microphone and a loudspeaker physically located within a room and each connected to a speakerphone signal processor. The speakerphone encounters two types of echo. The first type, known as electrical echo, is generated by the interfaces formed by various connectors of the speakerphone system (especially at the transmission line coupler (TLC)). A second type of echo is acoustic echo. Acoustic echo is generated by the room acoustics as the sound echoes off physical objects such as walls. Acoustic echo differs from electrical echo in that there are multiple echo paths. The “first attack” echo path represents the shortest distance from the loudspeaker to the microphone. Acoustic echo also differs from electrical echo because its duration is much longer. Depending on such factors as room size and building materials, an acoustic echo may not dissipate for several milliseconds. Thus, the implementation of an audio processor circuit that provides effective acoustic echo cancellation presents significant challenges. These challenges are even more complicated to address due to the inherent transit delay if the remote processor is a significant distance away from the signal conversion function.
The use of the same reference symbols in different drawings indicates similar or identical items.